A windmill whose wind-catching blades are made of sailcloth is conventionally known (e.g. a windmill of the non-patent document 1). This kind of windmill is referred to as a “sail-wing windmill”, which is advantageous in that (1) the blades can be made lighter; and (2) the blades can be made more easily, compared to a propeller windmill whose blades are made of rigid materials. Since the blades are not arranged closely in the conventional sail-wing windmill, there is a shortcoming that a high energy-convertion efficiency is not obtained when generating power using it. The reason is that the conventional sail-wing windmill is used in an open environment in which nothing exists around it. If the blades are arranged closely, a resistance generated by the windmill catching a wind becomes large, and the wind hit by the windmill escapes outside thereof without passing through gaps between adjacent blades of a windmill, a rotational efficiency becomes lower than a configuration in which the blades are arranged apart. Thus, the conventional sail-wing windmill is not suitable for a practical power generation apparatus.